MICROWAVE REPEATER RESEARCH 



227 



signed in accordance with the formulas given by a former member of this 

 laboratory.-^ In Fig. V-6, "a" shows a schematic diagram of such a trans- 

 former and "b" shows the equivalent tt network generally used. The equiv- 

 alent T network shown at "c" has also been used. In Fig. V-7 ''a" shows the 

 theoretical band-pass characteristic for this type of transformer with a 

 coupling coefficient of 0.5 which was the value used in most stages. The 

 circles indicate points measured on a typical transformer. This matched 

 network is relatively insensitive to small changes in capacitance so that 

 wherever it is used it is possible to change tubes without having to reahgn 

 the amplifier. The delay distortion for this type of network is, as shown by 



1 1 



(C) ''T" EQUIVALENT 



Fig. V-6. — The double tuned IF transformer. 



"b" in this figure, also relatively small. The gain per stage for a coupling 

 coefficient of 0.5 is approximately 6 db for 6AK5 vacuum tubes and about 

 12 db for the recently developed WE 404-A tubes. As shown by W. J. 

 Albersheim^^ it is possible to design circuits which will give more gain than 

 the ''matched"transformer, but only at the cost of increased sensitivity to 

 capacitance changes. For convenience the IF amplifier is usually divided 

 into a preamplifier closely associated with the receiving converter and a 

 main IF amplifier. 



Pre-amplifier — At the time work was begun, noise figures of the order of 



28 V. C. Rideout, "Design of Parallel Tuned Transformers", B. S. T. /., vol. 27, pp. 

 96-108, January 1948. 



